1. Field of Invention
This invention relates generally to heat transfer devices, and more particularly, to a multi-layer wick for a loop heat pipe.
2. Background of the Invention
Loop heat pipes are two-phase heat transfer devices that utilize the evaporation and condensation of a working fluid to transfer heat, and the capillary forces developed in fine porous wicks to circulate the fluid. Loop heat pipes are high efficient heat transfer devices that are used in space applications to transfer heat from one source to another using a fluid in a closed system. Loop heat pipes are different from conventional heat pipes, in that a wick structure is only required in the evaporator section. The wick structure, made of fine porous material, is typically comprised of a primary wick and a secondary wick that provide the driving force for circulating the operating liquid/vapor in the loop heat pipe.
FIG. 1 illustrates a schematic of a conventional loop heat pipe 2 with a wick structure having a primary wick 6 and a secondary wick 20, both made of metal. In loop heat pipe 2, heat is applied to an evaporator 4, in a loop arrangement, causing liquid to evaporate on a liquid/vapor interface within primary wick 6. Saturated vapor 8 flows through vapor grooves in evaporator 4 and merges into a vapor line 10 and a condenser 12 where heat is removed. In other words, the wick structure is used to drive the operating liquid/vapor in loop heat pipe 2 and provides a phase change interface for heat transfer.
Vapor 8 is collected by a system of grooves, which can be located in the wick structure, and flows down vapor line 10 to condenser 12, where it condenses as heat is removed within the condenser 12. The grooves allow vapor 8 to escape out of evaporator 4 into vapor line 10. A compensation chamber 14, at the end of evaporator 4, is designed to compensate the liquid supply of evaporator 4 and adjust the loop heat pipe operating temperature. The lower saturated pressure in compensation chamber 14 forces the condensed liquid to return to evaporator 4. The liquid/fluid then flows into a central pipe 18 where it feeds primary wick 6 and secondary wick 20. Excess fluid drains into compensation chamber 14.
The liquid in compensation chamber 14 and secondary wick 20 must be returned to primary wick 6 to close the cycle. Capillary forces accomplish this passively, sucking liquid back to the surface, just as water will be sucked up into a sponge.
FIG. 2 illustrates a graph of an occurrence of the heat leakage in loop heat pipe 2 of FIG. 1, verified by temperature measurements at various positions in loop heat pipe 2. Table 1 below identifies the positions in loop heat pipe 2 where the temperature was measured. As can be seen in FIG. 2, the greater the change in temperature (ΔT), the more heat leakage that results. For example, the change in temperature between the compensation chamber temperature (TC8) and the vapor line temperature (TC5) indicates a large heat leakage is occurring. This heat leakage is a result of the high thermal conductivity of primary wick 6 which is made of metal.
TABLE 1ThermocoupleTC-1TC-5TC-7TC-8TC-9PositionVapor inVapor outEvapo-CompensationLiquidcondenser(evaporator) 8rator 4Chamber 14line 1612
To reduce heat leakage, prior systems have substituted ceramic for the metal of primary wick 6. FIG. 3 illustrates an example of a wick structure where primary wick 6 is made of ceramic. Although using ceramic for primary wick 6 reduces heat leakage from evaporator 4 to compensation chamber 14, it also has the negative side effect of causing heat transfer resistance on the heating surface.
In FIGS. 4a-b, portions of loop heat pipes with the primary wick made of ceramic are illustrated. Each portion utilizes vapor grooves in different locations of the loop heat pipes to allow vapor to escape out of evaporator 22 into a vapor line (not shown). As can be seen in FIG. 4a, there is a large temperature difference (ΔT) between evaporator 22 and the vapor line, where groove 24 is located in evaporator 22. This large temperature difference indicates a large thermal resistance from the heating surface to the liquid/vapor interface due to the poor thermal conductivity of ceramic material. Similarly, with FIG. 4b, where groove 24 is located in wick structure 26, a large temperature difference (ΔT) resulting in a large heat leakage is caused by poor thermal conductivity of the ceramic material and the heat transfer across the dry-zone of ceramic wick.
FIG. 5 is a graph illustrating heat leakage of a loop heat pipe utilizing wick structure 26 of FIGS. 4a-b. Using a wick structure where the primary wick is made of ceramic decreases heat leakage, however, since the ceramic wick structure has a low thermal conductivity, there is a large temperature difference (ΔT) between evaporator 22 and vapor line 10.
In view of the above, what is needed is a multi-layer wick for a loop heat pipe that reduces heat leakage from the evaporator to the compensation chamber in a loop heat pipe, increases heat transfer and reduces heat transfer resistance within the ceramic wick.